Heretofore, as a technique of duplicating a disc base plate of an optical disc, a magneto-optical disc etc., there is proposed a photopolymer method, a so-called 2P method, which makes use of, for example, a synthetic resin curable on irradiation of UV rays (UV curable resin).
With the 2P method, a liquid UV curable resin is applied on the surface of a disc base plate, such as by screen printing, the disc base plate is placed on a stamper so that the UV curable resin is brought into intimate contact with the stamper, the disc base plate is pressed against the stamper by a roll, the UV curable resin is cured by irradiation of UV rays and the disc base plate is peeled from the stamper simultaneously with a cured resin layer for transferring the pattern of fine projections and valleys (grooves or pits) produced on the stamper surface.
The 2P method has an advantage that, by using the liquid resin, duplication may be made faithfully without stamper deterioration and, if a glass base plate is used as a base plate, a disc base plate may be obtained which is excellent in dimensional stability and thermal resistance and low in birefringence.
For duplicating the disc base plate by the above mentioned 2P method, an apparatus comprised of a stamper 101, on the surface of which a pattern of projections and valleys in formed, a rest block 102 for securing the stamper 101 , and a centering pin 104 for peeling a disc base plate 103, on which the pattern of projections and valleys on the stamper 101 has been transferred by photocuring of the UV curable resin, from the stamper 101 , as shown in FIG. 13.
The rest block 102 functions as a supporting block for securing the stamper 101 bearing a pattern of fine projections and valleys on its surface, and also for transferring the pattern to a UV curable resin layer, not shown, provided on the disc base pl ate 103. The block 102 has a through-hole 106, at a mid position thereof, through which a shank 105 of the centering pin 104 is introduced. The block 102 is supported on a base block 108 by means of support columns 107.
The centering pin 104 functions to secure the disc base plate 103 to a predetermined site on the stamper 101 and to peel off the disc base plate 103 from the stamper 101 simultaneously with the cured resin layer. Thus the centering pin 104 is comprised of a disc-shaped head 110 fitting into a centering hole 109 formed at a mid point of the disc base plate 103 for positioning and securing the disc base plate 103 at a predetermined position on the stamper 101 and a shank 105 which is slidable along the inner peripheral surface of the through-hole 106 of the rest block 102 as a guide.
The head 110 is of substantially the same shape as the centering hole 109 of the disc base plate 103, and is adapted to be engaged in the centering hole 109 from a side facing the stamper 101 . On the other hand, the shank 105 is passed through the through-hole 106 and is connected at the lower end thereof to a pneumatic cylinder 111 provided on the base block 108 so as to be moved vertically along the inner peripheral surface of the through-hole 106 as a guide. That is, the lower end of the shank 105 fits in a recess 114 of a connecting member 113 attached to the distal end of a piston rod 112 projecting from the pneumatic cylinder 112, and is secured thereto by a bolt 115 for interconnecting the shank 105 and the pneumatic cylinder 111.
For duplicating the disc base plate with the use of the above described apparatus, the disc base plate 103 coated with the UV curable resin is placed on the stamper 101 secured to the rest block 102 so that the UV curable resin is in intimate contact with the stamper 101. At this time, the disc base plate 103 is secured at a predetermined position on the stamper 101 with a head 110 of the centering pin 104 fitting in the centering hole 109.
The disc base plate 103 is pressed by a roll, not shown, onto the stamper 101, for uniformly charging the liquid UV curable resin uniformly into a space between the stamper 101 and the disc base pl ate 103.
UV rays are then irradiated from the disc base plate side for curing the UV curable resin.
As a result, a disc base plate is produced, on the UV cured resin layer of which the pattern of the projections and valleys of the stamper 101 have been transferred.
Meanwhile, with the above mentioned 2P method, the viscosity of the UV curable resin plays a crucial role and a satisfactory transfer depends on the value of the viscosity. For example, as shown in FIG. 12, the UV curable resin is gradually increased in viscosity and lowered in fluidity as the temperature is lowered. Hence, if the disc base plate 103 coated with the UV curable resin is pressed against the stamper 101 secured on the rest block 102 maintained at an ambient temperature, the UV curable resin is increased in viscosity and becomes poor in fluidity so that the resin can not be intruded satisfactorily into interstices between the projections and valleys on the stamper. The result is that faithfulness of duplication is deteriorated. Thus the UV curable resin has a range of viscosity which is suited to transfer and which needs to be maintained by some means or other.
In view of the above described status of the prior art, it is an object of the present invention to provide a disc transfer method whereby the pattern of projections and valleys of the stamper may be transferred satisfactorily to a UV curable resin layer.